This application relates to a system of employing one or more oxy-fuel burners in a regenerative furnace and methods for operating such burners in a regenerative furnace, to provide enhanced heat transfer while improving uniformity of heating and reducing potential formation of nitrogen oxides (NOx).
In a conventional regenerative furnace system, air-fuel burners are used in alternately firing pairs to recover energy from the flue gas of the first of the paired burners by preheating the air used during the operation of the second of the paired burners. In particular, a capacitive heat exchanger (e.g., refractory material) is used to absorb energy (as heat) from the flue gas while the first burner is firing and to then release that energy (as heat) to the air flow to the second burner, and vice-versa. While the each burner is firing, the other burner's air passage serves as the flue gas duct, and includes the capacitive heat exchanger, and the burners cycle back and forth periodically between firing as a burner and serving as the flue gas duct.
One example of the configuration of an end-port regenerative furnace 100 is shown in FIG. 9. The furnace is typically operated regeneratively using two burner ports 110, 130 alternately acting as burner and flue. The furnace is bounded a front wall 102, a back wall 120, and a first sidewall 114 and a second sidewall 134 each extending from the from the front wall 102 to the back wall 120, as well as a roof (not shown). A charge of material to be melted, such as glass or metal, is loaded into and positioned within the furnace 100.
In an end-port configuration, a first regenerator port 110 and a second regenerator port 130 are mounted in the front wall 102. In a first mode of operation, fuel and air are supplied to the first regenerator port 110 and combusted in the furnace 10, while hot combustion products are exhausted as flue gases via the air supply opening of the second regenerator port 130. The combustion gases travel a generally U-shaped path in the furnace 100. In a second mode of operation, fuel and air are supplied to the second regenerator port 130 and combusted in the furnace 100, while hot combustion products are exhausted as flue gases via the air supply opening of the first regenerator port 110. A flue (not shown) may be positioned in the furnace 100 to assist in creating a draft to maintain the U-shaped flow pattern. Furnace operation is alternated between the first mode and the second mode, such that the regenerator ports 110, 130 are cyclically operated as burner port and exhaust gas port, with a switch between the two modes occurring on a set time scale, such as every 10 to 30 minutes. A heat exchanger (also referred to as a regenerator) is positioned in each regenerator port 110, 130 so that air flowing inwardly through the regenerator port and exhaust flowing outwardly through the regenerator port pass across the heat exchanger, thereby preheating the incoming air with heat recovered from the departing exhaust.
Given the paired or alternate firing configuration of a regenerative furnace, it is sometimes difficult to optimally position the paired burners in a melting/heating process so as to obtain uniform heating. To ensure most flue gases exit through the second regenerator port while the first regenerator port is firing, a draft is introduced, which can result in short circuiting of flue gas and potential non-uniformities in energy distribution. For example, cold spots 122 may exist in the furnace 100 that result in extended cycle times. Such cold spots are common at the base of the U-shaped path, particularly in a furnace 100 that is relatively long compared to its width and compared to the regions reached by the flames emanating from the regenerator port burners 110, 130.
Strategically located oxy-fuel burners can provide an energy boost in the furnace, targeted at those cold spots, to improve heating uniformity, efficiency, and productivity, without significantly increasing the volume of flue gas. As with other uses of oxygen enrichment in an air-fuel furnace, increased combustion efficiency and higher flame temperatures may be beneficial. However, introduction of oxy-fuel burners into an air-fuel fired furnace may also detrimentally increase NOx emissions. See “Oxygen-Enhanced Combustion,” Charles E. Baukal, ed., CRC Press, 1998 (p. 48, FIG. 2.1) describing a peak in NOx near 45-50% oxygen in the oxidizer, with a drop in NOx at higher than 55% oxygen in the oxidizer as a result of lower nitrogen concentration in the oxidizer. Prior attempts to utilize oxygen enrichment in air-fuel fired regenerative furnaces have not overcome this problem.